Hi SleepyBug,
I am brand new here, but I thought maybe I could help.
I would tell your sister that a person who takes pain medicine when they actually have pain is not abusing the medicine. Plus, getting a fibro diagnosis does not mean she necessarily HAS TO go on pain meds. Perhaps she doesn't have fibro at all, but if it turns out that she DOES, maybe she can manage it with anti-inflammatory meds, or Lyrica, or Cymbalta...there's new treatments all the time. There's nothing that works for everybody, but there's lots of options.
Hope I'm not overstepping for a first post, but I do have a background in medicine, and I've been dealing with fibro for about 5 years now, and I thought those couple nuggets of wisdom might help to at least convince her to get checked-out, which seems to be the most important thing she can do for herself.
Good luck!